The answer is a. The number is to the thousandths place, and to find the tenth place you can only have one decimal place. To find the tenth, you look at the number in the hundreths place and that decides whether the tenth place gains a number or stays the same. If the the hundreths place is greater than five, you add one number on the tenths place. Since the hundreths place is greater than five, you add a number onto the tenths place. This makes the number 512.4